The invention relates to electrodeposition coating compositions (i.e., coating compositions that can be applied by cathodic electrodeposition ("CED") process) as well as to a process for the CED coating of substrates having a galvanized surface.
CED is a process in which coatings are applied to substrates where the substrate acts as the cathode. When substrates having an at least partially galvanized surface undergo CED coating, surface defects frequently form within the CED coating layer deposited due to the formation of fine hydrogen blisters on the substrate. The hydrogen blisters, particularly after the CED coating layer has been stoved, are visible as pinholes or so-called zinc cratering. The pinholes are usually still visible after subsequent coatings are applied. Thus, individual pinholes in the CED coating layer can only be eliminated by post-working such as sanding and post-coating with suitable repair coating agents. In this case, not only are quality deficiencies of the CED coating possible, but in particular the productivity of the CED coating process suffers. If the CED coating layer has a very large number of pinholes, post-processing is no longer possible. The defectively coated substrate then becomes scrap.
The problem of pinholes as outlined above becomes more pronounced as the deposition voltage is increased, in other words the number of pinholes per unit of area increases with the deposition voltage. Pinholes thus become a significant problem when coating substrates at high deposition voltage. CED coating of three-dimensional substrates such as, for example, automobile bodies, takes place at the highest possible deposition voltage in order to achieve the highest possible throwing power. Throwing power is understood to mean that ability of an electrodeposition coating agent to be deposited within voids of a three-dimensional substrate which is significant for effective corrosion protection. Accordingly, there is a need for CED coating compositions that produce substantially pinhole-free coatings, even at high deposition voltages.